wpf draw rectangle with mouse

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp22
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Canvas cvs;
        Point leftTopPt;
        Point rightDownPt;
        Shape currentShape;
        Rectangle rect;
        bool isDrawing = false;

        public MainWindow()
        {
            WindowState = WindowState.Maximized;
            InitializeComponent();
            Loaded += delegate
            {
                InitCvs(); 
            };
        }

        private void InitCvs()
        {
            cvs = new Canvas();
            cvs.Width = this.ActualWidth;
            cvs.Height = this.ActualHeight;
            cvs.Background = new SolidColorBrush(Colors.White);
            cvs.MouseDown += Cvs_MouseDown;
            cvs.MouseMove += Cvs_MouseMove;
            cvs.MouseUp += Cvs_MouseUp;
            this.Content = cvs;
        }

        private void Cvs_MouseUp(object sender, MouseButtonEventArgs e)
        {
            if (isDrawing)
            {
                rightDownPt = e.GetPosition(cvs);
                DrawRectangle();
                isDrawing = false;
                cvs.ReleaseMouseCapture();
            }
        }

        private void Cvs_MouseMove(object sender, MouseEventArgs e)
        { 
        }

        private void Cvs_MouseDown(object sender, MouseButtonEventArgs e)
        {
            if (!isDrawing)
            {
                leftTopPt = e.GetPosition(cvs);
                isDrawing = true;
            }
            else if (isDrawing)
            {
                rightDownPt = e.GetPosition(cvs);
            }
            cvs.CaptureMouse();
        }

        private void DrawRectangle()
        {
            rect = new Rectangle();
            rect.Width = rightDownPt.X - leftTopPt.X;
            rect.Height = rightDownPt.Y - leftTopPt.Y;
            rect.Fill = new SolidColorBrush(Colors.Red);
            rect.Stroke = new SolidColorBrush(Colors.Black);
            rect.StrokeThickness = 5;
            Canvas.SetLeft(rect, leftTopPt.X);
            Canvas.SetTop(rect, leftTopPt.Y);
            cvs.Children.Add(rect);
            this.Content = cvs;
        }
    }
}